At its inception radio telephony was designed, and used for, voice communications. As the consumer electronics industry continued to mature, and the capabilities of processors increased, more devices became available that allowed the wireless transfer of data between devices and more applications became available that operated based on such transferred data. Of particular note are the Internet and local area networks (LANs). These two innovations, among others, allowed multiple users and multiple devices to communicate and exchange data between different devices and device types. With the advent of these devices and capabilities, users (both business and residential) found the need to transmit data, as well as voice, from mobile locations.
The infrastructure and networks which support this voice and data transfer have likewise evolved. Limited data applications, such as text messaging, were introduced into the so-called “2G” systems, such as the Global System for Mobile (GSM) communications. Packet data over radio communication systems became more usable in GSM with the addition of the General Packet Radio Services (GPRS). 3G systems and, then, even higher bandwidth radio communications introduced by Universal Terrestrial Radio Access (UTRA) standards made applications like surfing the web more easily accessible to millions of users (and with more tolerable delay).
Even as new network designs are rolled out by network manufacturers, future systems which provide greater data throughputs to end user devices are under discussion and development. For example, the 3GPP Long Term Evolution (LTE) standardization project is intended to provide a technical basis for radiocommunications in the decades to come. Among other things of note with regard to LTE systems is that they will provide for downlink communications (i.e., the transmission direction from the network to the mobile terminal) using orthogonal frequency division multiplexing (OFDM) as a transmission format and will provide for uplink communications (i.e., the transmission direction from the mobile terminal to the network) using single carrier frequency division multiple access (FDMA).
Radiocommunication devices designed in accordance with the newer LTE standard, as well as those designed in accordance with other standards, may have to contend with high Peak to Average Power Ratio (PAPR) issues in their transmit chains. For example, radiocommunication devices which transmit on multiple carriers (frequencies) may generate compound signals having high PAPR which propagate through their transmit chain. In order to meet out-of-band emissions requirements, which may be imposed by the various radiocommunication standards, a power amplifier (and other components) which receives such compound signals and amplifies them prior to transmission needs to provide good linearity across a large dynamic range. This requirement makes power amplifiers used in such radiocommunication devices more expensive.
Accordingly, Peak Power Reduction (PPR) mechanisms and techniques have been implemented to reduce peak power in signals prior to their reaching, for example, the power amplifier. One approach which is sometimes used to reduce the peak power of an input waveform is to implement power clipping. In the power clipping approach, whenever the amplitude of the input signal is lower than a predetermined threshold, the input signal is passed to the output unchanged, and whenever the amplitude of the input signal exceeds the threshold, the output signal is clamped to the threshold level. Of course, the clipping operation destroys some of the information contained in the original signal. However, the user should be able to tolerate this loss of information as long as the threshold is kept sufficiently high.
The afore-described solutions for controlling peak power have generally been directed toward single or narrow band systems which use a single up-converter to present a signal to a single power amplifier. When a radiocommunication system uses two or more up-converters tuned to two or more frequencies, e.g., in a frequency separated system, the signals are typically combined and then amplified by a single power amplifier. In this case, the peak reduction achieved at baseband using the afore-described techniques does not materialize. This difference in the effectiveness of peak power reduction schemes between single or narrow band systems and frequency separated systems is primarily due to the time varying phase of the signals combined at RF relative to the phase under which peak power reduction was performed. Additionally, small amplitude differences in the different frequency bands' transmit up-conversion chains will have some impact on the peak power reduction but the major contributor is typically the different phase between the chains.
This problem has become more apparent with the advent of multi-band power amplifiers (currently in the research stage) where the simultaneous transmission of two widely spaced signals is becoming possible. As an example, consider a multi-band power amplifier capable of simultaneous transmission of a signal in band 3 (DL: 1805-1880) and band 1 (DL: 2110 to 2170). The worst case edge to edge frequency separation in this example is 365 MHz. Producing a combined signal at baseband and performing peak power reduction is very computationally expensive as there is a need to support the separation frequency to represent the combined signal using a high sampling rate. Accordingly, it would be desirable to provide an alternative that requires fewer implementation resources while achieving similar peak reduction as when processed at the higher rate.